Integration of CHASM/new Cherenkov light engines

[ikepc1]

We need to maintain the performance optimizations from CHASM which are necessary for functional objectives in upcoming missions. We need to track these quantities related to Cherenkov photons:

1. Timing
2. Atmospheric attenuation
3. Wavelength spectrum

And, these features related to the overall simulation:

1. Multiple detectors for one shower.
2. Interchangeable atmospheric models

[ikepc1]

The first step is to rewrite the initializer of EAS to reflect the option of different Cherenkov light engines, and write a CHASM wrapper. The Cphotang EAS class attribute will reflect the choice, and in the case of the CHASM choice, will be the CHASM wrapper. As long as its call signature is the same, the rest of the EAS class will remain unchanged. Only the essential upward going EAS parts of CHASM source code will be ported over.